Deposition systems and methods are commonly used to form layers of semiconductor materials, such as thin epitaxial films, on substrates. For example, a chemical vapor deposition (CVD) reactor system and process may be used to form a layer of semiconductor material such as silicon carbide (SiC) on a substrate. CVD processes may be particularly effective for forming layers with controlled properties, thicknesses, and/or arrangements such as epitaxial layers. Typically, in a deposition system, such as a CVD system, the substrate is placed in a reaction chamber within a susceptor and one or more process gases including reagents or reactants to be deposited on the substrate are introduced into the chamber adjacent the substrate. The process gases may be flowed through the reaction chamber in order to provide a uniform or controlled concentration of the reagents or reactants to the substrate.
A deposition system, such as a CVD reactor, may be used to form epitaxial layers of silicon carbide on a single crystal silicon carbide substrate having a predetermined polytype such as 2H, 4H, 6H, 15R, 3C and the like. The term “polytype” refers to the ordering and arrangement of layers of atoms within a crystal structure. Thus, although the different polytypes of silicon carbide are stoichiometrically identical, they possess different crystal structures and consequently have different material properties such as carrier mobility and breakdown field strength. The letters H, R and C refer to the general crystal structure of the polytype, namely, hexagonal, rhombohedral and cubic, respectively. The numbers in the polytype designations refer to the repetition period of layer arrangements. Thus, a 4H crystal has a hexagonal crystal structure in which the arrangement of atoms in a crystal repeats every four bi-layers.
FIG. 9 illustrates a hexagonal unit cell of a hypothetical crystal. The unit cell 60 includes a pair of opposing hexagonal faces 61A, 61B. The hexagonal faces are normal to the c-axis, which runs along the [0001] direction as defined by the Miller-Bravais indexing system for designating directions in a hexagonal crystal. Accordingly the hexagonal faces are sometimes called the c-faces which define the c-planes or basal planes of the crystal. Planes which are perpendicular to the c-plane are referred to as prismatic planes.
Silicon carbide possesses a number of advantageous physical and electronic characteristics for semiconductor performance and devices. These include a wide bandgap, high thermal conductivity, high saturated electron drift velocity, high electron mobility, superior mechanical strength, and radiation hardness. However, the presence of crystalline defects in silicon carbide films may limit the performance of electronic devices fabricated in the films, depending on the type, location, and density of the defects. Accordingly, significant research has focused on reducing defects in silicon carbide films. Certain defects, such as micropipes, are known to severely limit and even prevent device performance. Other defects, such as threading defects, are not considered to be electrically active, and therefore may not be detrimental to device performance, at densities normally found in epitaxial films.
For applications where a high voltage blocking capability is required (for example power switching applications), silicon carbide films are usually grown “off-axis.” That is, the substrate crystal is sliced at an angle that is slightly oblique to the normal crystal axis (called the c-axis). Taking for example a hexagonal polytype such as 4H or 6H, the oblique angle of the cut may be made in one of the standard crystallographic directions illustrated in FIG. 10, namely the [11 20] direction (towards a point of the hexagonal unit cell) or [10 10] direction (towards the center of a flat side of the hexagonal unit cell), or along a different direction.
Thus when an epitaxial layer is grown on the substrate, the deposited atoms bond to atoms at the exposed edges of the crystal layer steps, which causes the steps to grow laterally in so-called step-flow fashion. Step-flow growth is illustrated in FIG. 11. Each layer or step grows in the direction in which the crystal was originally cut off-axis (the [11 20] direction in the case illustrated in FIG. 11).
Surface morphological defects, i.e. defects in the shape of the surface of an epitaxial film, have been observed in silicon carbide epitaxial layers using conventional imaging techniques such as Transmission Electron Microscopy (TEM) and Nomarski microscopy. Surface morphological defects are generally considered to be caused by crystallographic defects in the material. Accordingly, research into the cause of surface morphological defects generally focuses on the physics of crystal growth.
Surface morphological defects are generally classified in accordance with their physical appearance. Thus, such defects have been classified as “comet”, “carrot” and “triangular” defects based on their appearance under a microscope. Carrot defects are roughly carrot-shaped features in the surface of the silicon carbide film. The features are aligned along the step flow direction of the film, and are characteristically longer than the depth of the layer in which they are formed. For example, a film having a thickness of 40 μm may contain carrot defects having a length of around 250 μm depending on the off-axis angle. The mechanism by which carrot defects form is currently unknown. Wahab et al. speculate that carrot defects are caused by perfect screw dislocations which are pinned to the surface of the substrate during growth, and that the dislocation dissociates into partials that propagate in the basal plane and form partial ledges in the film. Wahab et al., “Influence of epitaxial growth and substrate induced defects on the breakdown of 4H—SiC Schottky diodes,” Appl. Phys. Let. Vol. 76 no. 19, pp. 2725–2727 (2000). While Wahab et al. reported that carrot defects were not harmful to the absolute breakdown voltage of Schottky diodes, reverse leakage current was increased by the presence of carrot defects. Carrot defects may have deleterious effects on other device properties as well, particularly when the defect is located at a sensitive location, such as under the edge of a Schottky contact.
Thus, it would be desirable to reduce or minimize the concentration of carrot defects found in epitaxial films of silicon carbide.